Actuators are capable of changing form or shape in response to a stimulus or condition and, thereby, of affecting a transformation or action. Actuation is also accomplished by piezoelectric materials, ion-exchange resins and shape memory alloys. There is a need for reducing the weight of actuators, the noise associated with actuators and the currents and voltages required for actuator operation, and for increasing the strains that actuators can achieve. This is especially true for biomedical devices and equipment. For example, for some medical equipment directly in contact with patient tissues it is desirable to use actuators made from soft materials that do no damage to these tissues. In general, actuators which are light-weight and generate smooth motions are desired.
Recent advances in actuator technology include the use of polymers as a constituent of actuator devices. Of particular interest are those polymeric systems that operate in an electromechanical mode; that is, transform an electrical signal into a mechanical motion. The family of conjugated polymers which includes polyaniline and polypyrrole have been found to be particularly useful since for these materials volume change is an electrochemical phenomenon and, therefore, involves both electrical phenomena, such as resistance and capacitance, and chemical phenomena, such as oxidation and reduction. See, e.g., 1. Chiarelli, 1992; 2. Kaneto, 1996; and 3. Kaneto, 1999. The polymer in a conjugated, polymer-based actuator changes its oxidation level and thereby its volume with the application of an electrochemical potential to the actuator. When the oxidation level changes, ion transport into or out of the polymer, solvent transport in/out of the polymer, polymer chain configuration changes, and changes in interactions between polymer chains all contribute to volume change.
The electrochemical oxidation and reduction of emeraldine salt in aqueous acids is thought to be accompanied by proton, electron, and anion transfer. See, e.g., 4. Huang, 1986. The electrochemical oxidation of the polyaniline salt form to the pernigraniline form occurs with the removal of protons, electrons, and anions from the corresponding nitrogen atom. Similarly, the electrochemical reduction of the polyaniline salt form to the leucoemeraldine form is thought to occur below pH=0 with the addition of both protons and electrons to the corresponding nitrogen atom. Above pH=0, the reduction is thought to occur with the addition of electrons and the loss of anions to the corresponding nitrogen atom. The change in the polyaniline physical structure, from the phenyl to the quinoid structure of the benzene ring, is induced during oxidation with a corresponding reversal for reduction. The electrochemical oxidation of polypyrrole in aqueous solutions can be accompanied by anion and/or cation transport, as well as electron and solvent transport. Oxidation of polypyrrole occurs with the removal of electrons and either the insertion of anions or the de-insertion of cations, or both.
It is difficult to achieve high conductivity in conjugated polymers. High conductivity has been reported for pure polyacetylene with aligned polymer chains; however, this material is unstable in an air atmosphere. For the two materials typically used for actuators, polyaniline and polypyrrole, conductivities are generally low (less than a few tens of Siemens/cm (S/cm)). For example, the polyaniline fiber actuators reported in 5. Mazzoldi, 1998 had a conductivity of 3 S/cm. The highest previously reported conductivity for polypyrrole was 300 S/cm for polypyrrole doped with tosylate See, e.g., 6. Satoh, 1986).
Generally, polyaniline is prepared by mixing aniline, a protonic acid and a polymerization agent or initiator in aqueous media at a temperature above −5° C., and recovering the resultant product from the mixture. Representative examples of such preparatory methods and the polyanilines made therefrom are shown in U.S. Pat. Nos. 5,147,913 and 5,177,187. Specifically, a polymerization agent, such as ammonium peroxydisulfate, is presented in a protonic acid solution, such as a 1 m HCl solution, and this solution is added to aniline also dissolved in 1 M HCl. The resulting solution maintained at a chosen reaction temperature, and the precipitate thus formed is collected and washed with 1 M HCl to yield emeraldine hydrochloride. This salt may be converted to the emeraldine base by treatment with 0.1 M NH4OH. Films can be cast from the emeraldine salt by dissolving the salt in NMP. An emeraldine base film may be converted to the emeraldine salt form by immersing the film in an acid, such as 1 M HCl. This method produces films having low conductivity (<30 S/cm). Such films are too resistive to undergo complete and timely oxidation/reduction without a conductive layer in contact with one film surface. Stretching these materials is known to increase their conductivity still further.
If a voltage is applied to an electrode in electrical contact with one end of a conjugated polymer element, the voltage along the polymer decreases with distance from the electrode along the length of the conjugated polymer element if the conjugated polymer material has a low conductivity. Material further from the electrode experiences a smaller potential and is thus at a different oxidation level than material closer to the electrode. This is illustrated in FIG. 1. The drop in potential is schematically illustrated by the shading gradation and is also shown by the numerals. If the material is very resistive, then the material may not even be sufficiently electroactive to undergo oxidation/reduction reactions without a metal backing, film, or contact, along its length. A film with a metal layer backing is shown in FIG. 2. The drop in potential is again schematically illustrated by the shading gradation and is also shown by the numerals.
Because of the low conductivity (<400 S/cm for the discussion hereinbelow) of most conjugated polymer materials, actuators utilizing these materials generally include a metal backing layer (FIG. 2) (See, for example, 7. Kaneko, 1998; 8. Lewis, 1997; 9. Pei, 1993; and 10. Smela, 1995.). This metal layer can be generated on a film using a number of procedures: (1) thermal evaporation; (2) sputtering; (3) gilding; (4) casting the polymer onto a metal substrate or onto a substrate already coated with the metal; and (5) electrochemically depositing the polymer onto a metal. However, such metal layers are detrimental to the operation of the actuator because the metal (a) may corrode or react in the electrolyte; (b) may delaminate; and (c) may crack. Furthermore, such metal layers add processing steps and expense to the production of actuators.
Metal layers are also incompatible with linear actuation; that is, expansion and contraction along the length of the actuator, because such layers induce bending (FIG. 3). A bending actuator cannot generate as much force as a linear actuator because some of the energy is lost in the transformation from linear motion to angular motion and because of considerations of torque. Nevertheless, almost all conjugated polymer actuators make use of bending layers (See, for example, 8. Lewis, 1997; 9. Pei, 1993; 10. Smela, 1995; and 11. Otero, 1993). There are a few examples of linear conjugated polymer actuators; however these include a metal layer with the exception of: 5. Mazzoldi, 1998; 12. Kaneko, 1997; 13. Chiarelli, 1995; and 14. Takashima, 1995.
In Reference 12. Kaneko, 1997, polyaniline films were cast from solutions of emeraldine base (EB), and polyaniline rods were prepared from EB by gelation. The films and rods were then protonated using aqueous acids. The conductivity of these articles was not specified, indicating that this characteristic was not important. The highest conductivity reported to date for polyaniline films prepared in the EB form and subsequently protonated is 70 S/cm (See, 15. Monkman, 1991). With a conductivity of 70 S/cm, actuators experience a drop in applied potential as schematically illustrated in FIG. 1 hereof, and as actually shown by the present inventors for a film having a conductivity of 30 S/cm (See FIG. 6 hereof). The effects of low conductivity can be seen in the cyclic voltammogram of a rod shown in figure 1 of Reference 12. (Kaneko, 1997): that is, the broad peaks show that the entire film is not simultaneously undergoing oxidation or reduction, but rather that there is a distribution of times and potentials at which oxidation occurs, a result of low conductivity. For polyaniline films in contact with a platinum foil cycled in aqueous electrolytes, as is shown in the upper part of fig. 3 of Reference 16 (Kaneko, 1999), the peaks are narrow. A higher polymer conductivity permits the entire sample to be simultaneously oxidized and reduced, thereby increasing the actuation speed, which is beneficial for devices utilizing actuators. The effects of low conductivity can also be observed by comparing the measured currents for the free-standing rods of fig. 1 of Reference 12. (Kaneko, 1997) and those for films disposed on a platinum foil of fig. 3 of Reference 16 (Kaneko, 1999). In the former, the sample was 23 mm long and had a diameter of 3 mm, giving a volume of 163 mm3 (since the rod was formed from a 10% gel, the actual volume of polyaniline was 16.3 mm3), and the current was observed to be 12 mA. In the latter reference, the sample size was 3×3×0.03 mm3, or 0.27 mm3, and the current was observed to be 1.5 mA. The size ratio was therefore 16.3:0.27=60, but the current ratio was 12:1.5=8. Had rods having higher conductivity been used, the current would have been higher (more of the film would have been oxidized and reduced), and a larger length change would have been observed. The Kaneto patent (Reference 3. Kaneto, 1999) also teaches the use of metal electrodes to ensure that a uniform potential is applied to the actuator device.
Reference 13 (Chiarelli, 1995) describes the use of free-standing polypyrrole films in actuators. These films were commercially obtained and had an electrical conductivity of 150 S/cm. A sample length of 9 cm was used and low sample conductivity is evident from fig. 3 of Reference 13. The oxidation and reduction peaks of the free-standing films in acetonitrile were separated by more than 1 V; by contrast, when polypyrrole films are in contact with a metal, typical peak separation in this solvent is approximately 0.25 V, as shown in fig. 1 of Reference 11. Otero, 1993. The peaks in Reference 13 are also poorly defined, small, and broad in comparison with those shown in Reference 11. Large peak separation not only requires greater operating voltages, a disadvantage for actuator devices, but also results in a lower efficiency: the net amount of energy required to power the devices depends directly on peak separation. Therefore, higher conductivity is desirable. Reference 17. Della Santa, 1997, states: “the active part of the sample is confined to a 30 mm long portion, next to the gold electrode”, confirming the low conductivity of the films in Reference 13. However, neither Reference 13 nor Reference 17 teaches that a higher sample conductivity would be beneficial.
The linear actuator described in Reference 5 (Mazzoldi, 1998) had, as mentioned hereinabove, a conductivity of only approximately 3 S/cm. However, in order to improve the performance of the actuator, the authors suggest decreasing the fiber diameter; no mention was made of improving the conductivity.
Reference 14. Takashima, 1995 describes the measurement of the dimensional changes of stretched polyaniline films. No film conductivity was reported; however, the required slow cycling speed of 1 mV/sec is indicative of a low conductivity. Nor was conductivity discussed in Reference 18. Takashima, 1997; in both references, films were cast in the EB form and subsequently doped which results in low conductivity.
In organic electrolytes, oxidation/reduction will not take place if the conjugated polymer is an anion exchange material and the anion is insoluble in the organic electrolyte (See, e.g., 19. Okabayashi, 1987). During the preparation of highly conducting polyaniline actuators, one anion that may be used is 2-acrylamido-2-methyl-1-propanesulfonate (AMPSA); however, anions like AMPSA are insoluble in organic solvents such as propylene carbonate (PC), a desirable solvent for use in electrochemistry.
Conjugated polymers are capable of changing a wide range of properties in response to a stimulus or condition, including color, concentration of ions, hydrophobicity/hydrophilicity, and conductivity. Electrochemical devices rely upon the application of an electrochemical potential or a current to change a property of the conjugate polymer. In what follows, the term “electrochemical devices” is understood to refer to devices in which a potential or current is used to drive chemical reactions, particularly oxidations and reductions. Conjugated polymers have also been employed as constituents of electrochromic windows, supercapacitors, photoelectrochemical solar cells, and other devices. Typical conjugated polymers used in electrochemical devices include the polyanilines, polypyrroles, polythiophenes, and polyphenylene vinylenes. Again, as in the case for actuators, if the conjugated polymer material is resistive, the material may not even be sufficiently electroactive to undergo electrochemical reactions without a metal backing, or contact, along its length, and electrochemical devices utilizing conjugated polymers generally incorporate a metal layer. Such a metal layer may be detrimental to the operation of the device because the metal may corrode or react in the electrolyte. In addition, a noble metal such as gold or platinum is usually necessary, and these are expensive.
Accordingly it is an object of the present invention to is to provide a method for generating conjugated polymers having high conductivity.
Another object of the present invention is to provide actuators using conjugated polymers which do not require a metal layer along their length, thereby being useful for linear extension actuators.
Yet another object of the invention is to provide a method for treating highly conducting conjugated polymers such that they can be used in organic electrolytes.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.